The present invention relates to digitally compensating linear distortion in a transmitter (Tx) of a modem and more particularly to digitally compensating linear distortion in a cable modem Tx.
“Modem” stands for modulator/demodulator. Modems may be used to transmit and receive data via telephone lines, cable systems and the like. A cable modem or a similar device may be used to provide high-speed access to the Internet or other computer network. A downstream direction refers to data flow to a cable modem (from a cable head-end or cable modem termination system (CMTS)), and an upstream direction refers to data flow from a cable modem (to a CMTS).
A cable plant provides downstream transmission in the higher frequency range, and upstream transmission in the lower frequency range. Thus higher frequency components of cable modem Tx signals are undesirable and should be minimized, so as not to impair downstream signals. To do so, they are attenuated with a few passive low-pass filters (LPF). But the LPF distorts the signal in the pass-band; also, there are additional sources of linear distortion in the Tx path, namely, a digital-to-analog converter (DAC) and power amplifier. A non-distorting path would have flat frequency response (FR); FR magnitude tilt and FR delay tilt are measures for linear distortion.
Today's requirements on FR cannot be provided with a low-cost LPF. Thus, the equalization (i.e., tilt compensation) problem arises. Presently, an analog equalizer is used to compensate for FR tilt. However, it is not a cost effective solution; the combination of an equalizer and LPF is the same as a high quality LPF. A better LPF requires more accurate components (and more components), raising cost and complexity.
On another hand, a high quality LPF makes more work than needed. The upstream spectrum spans the range of Fmin=5 to Fmax=42 Megahertz (MHz) in cable plants complying with the Data-Over-Cable Service Interface Specifications (DOCSIS), version 2.0, published Dec. 31, 2001, which define interface requirements for cable modems.
A cable modem never uses all of the upstream range at once. The spectrum is divided into a few channels, with the widest one occupying 6.4 MHz; in every given moment the cable modem is programmed to use one of the channels. But a LPF is non-programmable, so its FR must be flat in the whole upstream range.
Thus a need exists to equalize the Tx path “in band” (i.e., in the current channel only), instead of equalizing it in the whole upstream range at once, as a high quality LPF does.